The present invention relates to metal matrix composites having improved fatigue properties.
It is well known in the art to improve one or more properties (e.g. strength, toughness, wear resistance, fatigue resistance, etc.) of metals and alloys by inclusion of reinforcing particles therein, either as dispersed particles or as one or more preformed inserts formed from particulates. The reinforcing particles can comprise elongated fibers, rounded particles, and other particle shapes. Such reinforced metals or alloys are referred to as metal matrix composites (MMC""s) wherein the metal or alloy provides the matrix for the reinforcing particles. Metal matrix composites having various reinforcing particles and matrix metals/alloys are described in U.S. Pat. Nos. 5,679,041 and 5,588,477.
An object of the present invention is to provide a metal matrix composite and method of making same that incorporates fibrous reinforcements together with non-fibrous particles having certain particle dimensions controlled in a manner that improves fatigue properties of the composite.
The present invention provides a metal matrix composite and method of making same wherein a metallic matrix includes fibrous reinforcements that are present in the matrix together with non-fibrous particles typically present with the fiber reinforcements as incidental or pest particles, although they may be present intentionally as well. The non-fibrous particles have a shell morphology and can exist in the matrix as harmless shells filled with the matrix metal/alloy and also as harmful hollow shells exerting an adverse affect on fatigue properties of the composite. In accordance with the present invention, the non-fibrous particle shells have shell wall thicknesses controlled thick enough to improve fatigue properties of the composite, rather than detract from the fatigue properties. In particular, the fibrous reinforcements and non-fibrous particles are formed in a manner that the non-fibrous particle shell walls are in a range of thicknesses found to improve fatigue properties of the composite.
In an illustrative embodiment of the present invention offered for purposes of illustration only and not limitation, the fibrous reinforcements and non-fibrous particles comprise the same ceramic material, such as aluminosilicate, melt spun in a manner that provides predominantly relatively thick-walled non-fibrous particles having a shell wall thickness, t, and external shell radius, a, such that the ratio, t/a, is greater than about 0.2. When such fiber reinforcements and relatively thick-walled non-fibrous particles are incorporated into an aluminum based matrix, the resulting composite exhibits improved fatigue properties as compared to a similar composite having like fiber reinforcements but including thinner-walled non-fibrous particles.
The present invention envisions in a particular illustrative embodiment an improved piston for use in an internal combustion engine wherein the piston includes at least at a local region the above metal matrix composite structure to impart improved fatigue properties to the region. The reinforced region may include the crown, dome or ring groove region of the piston.